1. Field of the Invention
The present invention generally relates to dynamic seals of the type used in turbomachinery. More particularly, this invention relates to a seal assembly comprising a seal and a platform mounted, respectively, to non-rotating and rotating components of a turbomachine, wherein the platform is adapted to promote the dissipation of heat produced by rubbing contact between the seal and platform.
2. Description of the Related Art
Labyrinth-type packings and brush seals are widely used in steam turbines and in aircraft and industrial gas turbines to provide dynamic seals between the rotating and static turbine components, such as the rotor and diaphragm inner web of a steam turbine. Traditional labyrinth packing comprises a series of teeth that project radially inward from the circumference of a static component and toward but out of contact with the rotary component, thereby defining a series of partial barriers that create a tortuous axial flow path immediately adjacent the surface of the rotary component. Brush seals typically comprise metal bristles that, similar to the teeth of a labyrinth packing, project radially inward from the circumference of a static component toward a rotary component. In contrast to labyrinth packings, brush seals are normally intended to be in rubbing contact with the adjacent circumferential surface of the rotary component, creating a substantially continuous barrier to flow around the circumference of the rotary component. In this regard, brush seals provide a more effective barrier to secondary flow losses, i.e., provide better sealing performance, as compared to labyrinth packings, and therefore have the potential for significantly improving section performance.
The rotors of aircraft and industrial gas turbines are relatively stiff, and as a result their dynamic behavior is not generally affected by rubbing contact with a brush seal. In contrast, the rotor of a steam turbine typically includes a continuous solid shaft to which buckets are attached. Impulse-type steam turbines typically operate above the rotor""s first bending critical frequency, and often near the second bending critical frequency. It has been shown that the rubbing contact between a brush seal and the rotor of a steam turbine can magnify rotor vibration through the first and second critical speeds of a rotor, resulting in unacceptable radial rotor movement. It is believed that this effect is particularly likely to occur if the rotor is bowed as a result of thermal, dynamic or manufacturing circumstances. More particularly, the friction resulting from the rubbing contact locally increases the surface temperature of the rotor, leading to nonuniform surface temperatures along its circumference. Because high (proud) spots of a bowed rotor are particularly prone to heating in this manner from more intense rubbing contact, the localized heating caused by brush seals can further increase bowing in a rotor as a result of nonuniform thermal expansion about the rotor circumference, thereby exacerbating vibration and rotor dynamics concerns.
Various solutions to the above problem have been proposed. For example, brush seals have been installed in steam turbines with an initial clearance sized to ensure that their bristles do not contact the rotor during start-up, when there is a lack of adequate cooling flow that would otherwise reduce the degree of localized heating. While such an approach can eliminate or at least significantly reduce rotor dynamics concerns associated with brush seals, sealing performance is compromised. In commonly-assigned U.S. Pat. No. 6,168,377, a brush seal assembly includes an annular-shaped insert located in a circumferential groove in the rotor surface. The bristles of the brush seal are aligned to be in rubbing contact with the insert, such that heat generated by rubbing contact with the bristles is better distributed and dissipated by the insert. Another solution posed by commonly-assigned U.S. Pat. No. 6,517,314 is to position the brush seals in rubbing contact with platforms formed on the bucket dovetails, such that rubbing contact occurs at a location spaced radially outward from the outer diameter of the rotor shaft, thus eliminating localized heating at the rotor surface.
While the above approaches to reducing localized rotor heating have been proven to be effective, the use of brush seals in steam turbines has remained limited by rotor-dynamic constraints associated with localized heating of the rotor. As a consequence, in the diaphragm packing area of a steam turbine, brush seals have been limited by the number of stages in which they can be installed without inducing unacceptable radial rotor movement. The result is decreased section efficiency due to the increased secondary flow losses associated with the labyrinth packings that must be used instead of brush seals.
In view of the above, it would be desirable if alternative approaches were available for reducing vibration and rotor dynamics concerns arising from the localized heating caused by brush seals, such that brush seals can be used in essentially all stages within the diaphragm packing area of a steam turbine.
The present invention provides a seal assembly and method therefor that is capable of significantly reducing vibration and rotor dynamics concerns that arise in turbomachinery, such as steam turbines, as a result of localized heating caused by seals in rubbing contact with a rotary member of a turbomachine. The invention enables such seals, including brush seals, to be installed in essentially all stages within the diaphragm packing area of a steam turbine.
In view of the above, the present invention finds application with a turbomachine having a rotary member rotatable about an axis and a stationary member circumscribing the rotary member. The rotary member has an outer circumferential region disposed in a plane normal to the axis thereof and the stationary member having a portion radially extending toward the rotary member so that the stationary member has a radially inward surface region facing the outer circumferential region of the rotary member. The seal construction of this invention entails a seal assembly comprising an annular-shaped platform on the rotary member so as to be disposed at the outer circumferential region thereof. The platform is supported radially outward from the rotary member so as to project radially outward beyond axially-adjacent outer surface regions of the rotary member. Furthermore, the platform has a contact region radially separated from the rotary member by a cavity therebetween. A sealing member is disposed at the radially inward surface region of the stationary member, with a portion of the sealing member being in rubbing contact with the contact region of the platform. As a result of being spaced apart from the rotary member, with a cavity axially separating the contact region of the platform from the rotary member, the platform is effective to distribute and dissipate heat generated by rubbing contact with the sealing member.
The present invention also provides a method of providing a seal assembly for a turbomachine of the type described above. The method comprises the steps of providing the annular-shaped platform on the rotary member so as to be disposed at the outer circumferential region thereof, such that the platform is supported radially outward from the rotary member, projects radially outward beyond axially-adjacent outer surface regions of the rotary member, and has a contact region radially separated from the rotary member by a cavity therebetween. The sealing member is provided at the radially inward surface region of the stationary member, so that a portion of the sealing means is in rubbing contact with the contact region of the platform. Again, as a result of being spaced apart from the rotary member with a cavity therebetween, the platform is effective to distribute and dissipate heat generated by rubbing contact with the sealing member as the rotary member rotates about its axis.
In view of the above, it can be seen that a significant advantage of this invention is that the platform is configured to promote the distribution and dissipation of heat generated as a result of rubbing contact with a seal. More particularly, the platform is able to conduct heat axially from the contact area toward the ribs, thereby better distributing frictional heating that would otherwise cause a localized rise in temperature in a relatively limited surface region of a rotary member, in which case nonuniform surface temperatures and bowing of the rotary member could occur. Vibration and rotor dynamics concerns can be sufficiently reduced with the present invention to the extent that brush seals can potentially be used in every stage within the diaphragm packing area of a steam turbine, limited only be the particular brush seal design. The platform is preferably fabricated to be symmetrical about the axis of the rotary member, so as not to affect vibrational characteristics. The platform can also be configured to be well-adapted for field installation, as well as for use in original production.
Other objects and advantages of this invention will be better appreciated from the following detailed description.